For many years, a number of modulation techniques have been used to transfer data from a source to a destination. One type of modulation technique is referred to as multi-carrier modulation (MCM). In accordance with MCM, data is split into several data components and each of these data components is transmitted over separate carriers so that each individual carrier has a narrower bandwidth than the composite signal. In general, a “carrier” (sometimes referred to as a “tone”) is an electromagnetic signal transmitted generally at a steady base frequency of alternation on which information can be imposed. Of course, when used in connection with fiber optic medium, the carrier may be a light beam on which information can be imposed. The frequency range of the carrier may be referred to as a “frequency slot” or “frequency bin”.
Currently, there exist a number of multi-carrier modulation schemes such as Orthogonal Frequency Division Multiplexing (OFDM) for example. OFDM subdivides the available spectrum into a number of narrow band channels (e.g., 50 channels or more). The carriers for each channel may be spaced close together and each carrier is configured to be orthogonal to its adjacent carriers. This orthogonal relationship may be achieved by setting each carrier to have an integer number of cycles over a symbol period. Thus, the spectrum of each carrier has a null at the center frequency of each of the other carriers in the system. This results in no interference between the carriers, allowing them to be spaced as close as theoretically possible.
In particular, OFDM modulation is performed by encoding data onto individual carriers in the frequency domain. This encoding may be accomplished by a Fast Fourier Transform (FFT) engine. For instance, DQPSK modulation involves two-bits of data being encoded on to each carrier. An inverse FFT (IFFT) is performed on the set of frequency carriers, converting to the time domain and producing a single OFDM symbol. The OFDM symbol is then sent through a channel using a digital-to-analog converter (DAC).
Normally, receivers for MCM systems include an analog-to-digital converter (ADC) that is used to sample the data and route the sampled data to a FFT engine. The FFT engine detects data bits placed on a carrier by computing the phase of successive complex differential signals. Such computations are time consuming and require substantial processing power.
After detection, it is sometimes desirable for the MCM system to estimate the quality of the carrier in order to determine if the carrier transferring the data is unreliable. A carrier may be deemed “unreliable” where it is experiencing unfavorable channel characterizations (e.g., fading, thermal noise, high degree of interference, etc.). One type of estimation scheme is referred to as “data based channel estimation” in which knowledge of the test data is required. Pilot tones are sometimes used as the test data.
Another type of estimation scheme is referred to as “blind channel estimation,” where the receiver has no knowledge of the transmitted data. This lack of knowledge makes it more difficult to accurately estimate whether a carrier of a frequency band is reliable.